1. Field of the Invention
This invention relates to a mesh grid for a fluorescent display tube in which a fluorescent substance is prevented from adhering to the mesh because of deformation due to generated heat, vibration or the like.
2. Description of the Prior Art
Fluorescent display tubes having a general construction have various types of electrodes and the like in the interior of an envelope held in a highly vacuum state. For example, an anode display unit including usually a plurality of anode segments coated with a fluorescent substance is formed inward above the anode substrate which is one component of the envelope. Inside of the envelope, a grid for controlling electrons is provided over the anode display unit and a cathode is provided further over the grid as an electron source.
As the grid, a so-called spacer-frame-type grid, a so-called mesh grid and the like have been well known. In the spacer-frame-type grid, a mesh which is a different body from a frame is installed inside of the frame. In the case of the spacer-frame-type, a frame member called a spacer frame in which the frame, a mesh, other electrodes and the like are mounted is positioned at an appropriate position on the anode substrate and then the aforementioned anode and other components are disposed at predetermined positions on a top surface of the anode substrate. Then, a box-shaped container portion is fixed to the top surface of the anode substrate so as to cover the components. A lead connecting the space frame and the frame is fixed to a position in which the envelope is sealed.
Further, as the aforementioned grid, a mesh grid 100 shown in FIGS. 7 (a) and 7 (b) has been well known. This mesh grid includes an edge portion 101 and a mesh portion 102 which are integrally formed from a piece of metallic material by for example, etching or the like. A so-called over-mounting-type structure is utilized to install this mesh grid inside of the envelope. Here, the mesh portion 102 is allocated over the anode display unit 104 on the anode substrate 103 and then a leg portion 105 formed on the edge portion 101 of the mesh grid 100 is directly fixed to the anode substrate 103 by an adhesive.
Recently, the fluorescent display tube has been widely used in vehicles. As the fluorescent display tube is to be mounted on a vehicle, the mesh grid is often applied because it can be allocated in a relatively free shape or arrangement corresponding to a high density display pattern. The mesh grid is more likely to be deformed than the grid installed on the frame like in the aforementioned spacer-frame-type. Thus, the mesh thereof is sometimes deformed toward the anode substrate 103 as shown in FIG. 8 due to vibration caused when a vehicle is running or when heat is generated when the fluorescent display tube is lit, such that it is in contact with the fluorescent substance of the anode display unit 104. In the fluorescent display tube mounted on a vehicle, a distance between the mesh grid and the anode display unit is set relatively small to secure visibility under a strong daylight. Thus, this is one reason why the mesh is likely to go into a contact with the anode display unit.
If the mesh grid is in contact with the fluorescent substance, the fluorescent substance adheres to a rear side of the mesh portion. Then, the fluorescent substance adhering to the mesh grid emits light when this fluorescent display tube is driven, thereby disturbing indications. As described before, the fluorescent display tube mounted on a vehicle needs to secure visibility under a strong daylight. For this reason, in the daytime, an anode voltage is increased to intensify brightness. Thus, this unnecessary light caused by the flouorescent substance adhering to the mesh grid is not so visible. However, because light emitted in the daytime is too strong for the eyes of a driver, the anode voltage of the fluorescent display tube of a display panel is decreased at night time interlinked with a switch-on of head lamps or the like. In this case, a dimming light emission method for decreasing the brightness of the anode display unit is utilized. Thus, if the fluorescent substance adheres to the mesh grid of the fluorescent display tube mounted on a vehicle, the unnecessary light emission is particularly visible at night time thereby disturbing visibility of the display panel.
A proposal for preventing the aforementioned deformation of the mesh grid has been made in Japanese Published Utility Model No. Hei 1-41154 (Japanese Utility Model Laid-Open No. Sho 60-96763). According to this proposal, as shown in FIGS. 9 (a) and 9 (b), a portion having no mesh is left on a periphery or inside of the mesh grid and then nail shaped auxiliary tabs are formed on that portion by punching out using a press, so as to be used as a support if the mesh grid is deformed toward the anode substrate.
However, such a support structure needs a wide area in the small mesh grid. Thus, it is difficult to apply this support structure particularly to the fluorescent display tube having a high display density, mounted on a vehicle. Further depending on a shape of the segment of the anode display unit, this structure may not be sufficient.